This invention relates to current meters, which are devices for measuring currents in an aquatic medium, more particularly for measuring the velocity of marine currents, by the Doppler effect.
The measurements are made in a moving boat and the speeds of the currents are determined practically from the surface to great depths which may exceed 1000 m.
Knowledge of the profile of currents as a function of depth is important for navigational estimates, for navigating submarines, in petroleum and mining exploration on the high seas and in geophysical studies and the study of the environment.
One type of current meter, known as a conventional type, such as those comprising a screw or the electromagnetic type, having to be placed in the medium in order to measure the current, and unless the number of pickups is multiplied they can only make point measurements. It is known that, in order to obtain a quasi-instantaneous measurement of the profile of currents, it is necessary to use another type of current meter, i.e. Doppler effect current meters. For this, sonars are used the transmission axes of which are inclined relative to the vertical. The echos obtained are due, on the one hand, to the volume reverberation and, on the other hand, to the reflection by the sea bed.
The volume reverberation echos undergo a variation in frequency because of the Doppler effect, depending on the relative velocity of the boat in relation to the current at the point measured. The depth corresponding to the measuring point is deduced from the return time.
The absolute velosity of the boat is calculated from the variation in frequency by the Doppler effect of the echo produced by the sea bed.
The measurements are taken in two perpendicular vertical planes, thus enabling the boat/current velocity and boat/sea bed velocity vectors to be established.
The current/sea bed speed vector is deduced by subtaction from the preceding vectors. If this operation is carried out for all the volume reverberation echos at the different depths, the profile of the current velocity vectors is obtained.
To obtain sufficient power for the reverberation echos, experience has shown that high frequencies (.perspectiveto.50 kHz) must be used; on the other hand in order to obtain echos on sea beds of the order of 3,000 m deep, taking the propagation losses into account, low frequencies (.perspectiveto.10 kHz) have to be used.
It is known to transmit and receive simultanously at one low frequency and at one high frequency. The low frequency being used for the sea bed echo and the high frequency for the reverberation echos.
However, the transmission of a low frequency makes it necessary to use transducers with dimensions of the order of one meter for an angle of opening of the radiation diagram of 7.degree., which would result in a prohibitively large bulk.
It is known too to detect targets in the sea bed, with a sonar transmitting two high frequencies and receiving echos of a low frequency, differences of the two high frequencies, generated by the non-linear effects of sea water.